It is known to use an electric drive motor 1 to generate a traction force to propel a vehicle, such as an automobile. A quadrant of the electric drive motor 1 is shown in FIG. 1. The electric drive motor 1 is a permanent magnet synchronous motor comprising a rotor 2 and a stator 3. The rotor 2 comprises a plurality of laminations of a ferromagnetic material to form a rotor iron. First and second magnets 4, 5 are embedded in the rotor to form a plurality of rotor poles 6. The first and second magnets 4, 5 are permanent magnets (PM) and generate a magnetic flux. An assumed reference frame is shown in FIG. 1, where the rotor pole 6 has a d-axis aligned to the permanent magnet (PM) flux, and a q-axis arranged transverse to the direction of the rotor pole 6 (i.e. transverse to the d-axis). The angular extent of each rotor pole 6 (i.e. the included angle between the +q-axis and the −q-axis) is referred to herein as the pole step. An air gap is maintained between the rotor 2 and the stator 3.
The first and second magnets 4, 5 in each rotor pole 6 are arranged in first and second layers L1, L2 one above the other in a radial direction. As shown in FIG. 1, a pair of said first magnets 4-1, 4-2 is arranged in the first layer L1 and one of said second magnets 5 is arranged in the second layer L2. The first and second magnets 4, 5 in the first and second layers L1, L2 are arranged symmetrically about, and substantially perpendicular to the d-axis. The stator 3 comprises a plurality of slots 7 extending radially inwardly to support coil windings 8. The electric drive motor 1 comprises forty-eight (48) slots 7 and eight (8) rotor poles 6. By energising the coil winding 8, a torque is generated to drive the rotor 2.
The rotor 2 comprises a plurality of first and second cavities 9, 10 associated with the first and second magnets 4, 5 in said first and second layers L1, L2 respectively. The first and second cavities 9, 10 are arranged to influence the magnetic flux generated by the first and second magnets 4, 5. In the illustrated arrangement, the first and second cavities 9, 10 are elongated and extend at an acute angle relative to the d-axis. The first cavities 9-1, 9-2 extend outwardly from the lateral ends of the first magnets 4-1, 4-2 in the first layer L1; and the second cavities 10-1, 10-2 extend outwardly from the lateral ends of the second magnet 5 in the second layer L2. The first and second cavities 9, 10 are formed within the rotor 2 such that the outer surface of the rotor 2 is continuous.
It will be appreciated that the distribution of ferromagnetic material in the rotor 2 and the stator 3 is uneven due to the presence of the first and second magnets 4, 5 in the rotor 2; the slots 7 in the stator 3; and the first and second cavities 9, 10. As a result, the torque and voltage characteristics of the electric drive motor 1 contain harmonic components. The voltage harmonics are developed by the harmonics of flux density distribution in the air gap, the harmonic order number being equivalent to the harmonic order number of a voltage high harmonic. The torque harmonics differ from the voltage harmonics, since they are developed by the interaction of harmonic components. For example the 6th torque harmonic component is the result of the interaction of the base harmonic with the 5th and 7th order harmonics of flux density distribution (6=5+1=7−1).
The interaction of the harmonic components results in torque ripples which can result in generation of noise and vibrations when the electric drive motor 1 is operating. These torque ripples are typically undesirable.
It is against this background that the present invention has been conceived. At least in certain embodiments, the present invention seeks to reduce or ameliorate at least some of the aforementioned shortcomings.